T. Valdemarsen*, E. Kristensen, M. Holmer

ABSTRACT: The effects of organic enrichment on sediment biogeochemistry were studied in sediment mesocosms with indigenous populations of infauna. Two types of sediments, differing in content of reactive Fe, were subjected to weekly pulses of labile organic matter (fish feed), corresponding to an average organic loading of ~350 C m–2 d–1 and ~50 mmol N m–2 d–1 for a period of 65 d. The aims of this experiment were 3-fold: (1) to study the metabolic capacity for organic C and N mineralization at the sediment–water interface; (2) to study the impact of organic enrichment on spatial and temporal sulfide accumulation; and (3) to estimate the importance of Fe-driven sulfide buffering for sulfide accumulation. Organic enrichment stimulated solute exchange between sediment and water within 10 d, resulting in 5 to 8 times stimulated O2 uptake and CO2 efflux, indicating a fast response time of surface-associated microbial consortia. Both 35S-SO42– radiotracer essays and sulfide microprofiles confirmed that added organic matter was primarily oxidized by sulfate reduction. However, despite high sulfate reduction of up to 70 mmol m–2 d–1, sulfide only accumulated to 1–2 mM in the upper 2 cm of enriched cores. In Fe-poor sediment, produced sulfide was reoxidized with O2 or NO3– at the sediment–water interface, whereas in Fe-rich sediment, sulfide was oxidized with O2, NO3–, or Fe oxides in a <2 mm thick suboxic zone or precipitated with dissolved Fe2+. By the end, up to 92 and 100% of added organic C and N, respectively, was mineralized, suggesting a high metabolic capacity for organic matter mineralization in faunated sediments. Furthermore, whereas sulfide precipitation was insignificant in low-Fe sediment, up to 63% of sulfide precipitated with Fe2+ in high-Fe sediment, suggesting a high capacity for sulfide mitigation in faunated Fe-rich sediment.